Simple Dictionary

Simple accretive dictionaries have an interface nearly equivalent to dict. Because of their accretive nature, they can be useful as registries for extensions, handlers, and plugins, such that something that is registered is guaranteed to remain registered throughout the lifetime of the registry.

>>> from accretive import Dictionary

Let us illustrate this use case by first defining some handlers to register.

>>> def csv_reader( stream ): pass
...
>>> def env_reader( stream ): pass
...
>>> def hcl_reader( stream ): pass
...
>>> def ini_reader( stream ): pass
...
>>> def json_reader( stream ): pass
...
>>> def toml_reader( stream ): pass
...
>>> def xml_reader( stream ): pass
...
>>> def yaml_reader( stream ): pass
...

Initialization

Simple dictionaries can be initialized from zero or more other dictionaries or iterables over key-value pairs and zero or more keyword arguments.

>>> readers = Dictionary( { 'csv': csv_reader }, ( ( 'json', json_reader ), ( 'xml', xml_reader ) ), yaml = yaml_reader )
>>> readers
accretive.dictionaries.Dictionary( {'csv': <function csv_reader at 0x...>, 'json': <function json_reader at 0x...>, 'xml': <function xml_reader at 0x...>, 'yaml': <function yaml_reader at 0x...>} )

Immutability

Existing entries cannot be altered.

>>> readers[ 'xml' ] = toml_reader
Traceback (most recent call last):
...
accretive.exceptions.EntryImmutabilityError: Cannot alter or remove existing entry for 'xml'.

Or removed.

>>> del readers[ 'xml' ]
Traceback (most recent call last):
...
accretive.exceptions.EntryImmutabilityError: Cannot alter or remove existing entry for 'xml'.

(Seems like XML is here to stay.)

Updates

However, new entries can be added individually or in bulk. Bulk entry is via the update method.

>>> readers.update( ( ( 'env', env_reader ), ( 'hcl', hcl_reader ) ), { 'ini': ini_reader }, toml = toml_reader )
accretive.dictionaries.Dictionary( {'csv': <function csv_reader at 0x...>, 'json': <function json_reader at 0x...>, 'xml': <function xml_reader at 0x...>, 'yaml': <function yaml_reader at 0x...>, 'env': <function env_reader at 0x...>, 'hcl': <function hcl_reader at 0x...>, 'ini': <function ini_reader at 0x...>, 'toml': <function toml_reader at 0x...>} )

Note

The update method returns the dictionary itself. This is different than the behavior of dict, which returns None instead. Returning the dictionary is a more useful behavior, since it allows for call chaining as a fluent setter.

Copies

Copies can be made which preserve behavior and data.

>>> dct1 = Dictionary( answer = 42 )
>>> dct2 = dct1.copy( )

Copies can also be made which preserve behavior but replace data. These are made using the with_data method, which creates a new dictionary of the same type but with different data. This is particularly useful with producer and validator dictionaries (see below) as it preserves their behavior:

>>> base = Dictionary( a = 1, b = 2 )
>>> new = base.with_data( x = 3, y = 4 )
>>> new
accretive.dictionaries.Dictionary( {'x': 3, 'y': 4} )

Comparison

The copies are equivalent to their originals.

>>> dct1 == dct2
True

And to instances of other registered subclasses of collections.abc.Mapping which have equivalent data.

>>> dct2 == { 'answer': 42 }
True

Modifying a copy causes it to become non-equivalent, as expected.

>>> dct2[ 'question' ] = 'is reality a quine of itself?'
>>> dct1 == dct2
False
>>> dct2 != { 'answer': 42 }
True

Access of Absent Entries

As with dict, a missing entry will raise a KeyError.

>>> dct1[ 'question' ]
Traceback (most recent call last):
KeyError: 'question'

And, like dict, the get method allows for “soft” accesses which provide a default value if an entry is missing.

>>> dct1.get( 'question' )
>>> dct1.get( 'question', 'what is the meaning of life?' )
'what is the meaning of life?'

Views

The usual methods for producing views on items, keys, and values exist.

>>> tuple( readers.keys( ) )
('csv', 'json', 'xml', 'yaml', 'env', 'hcl', 'ini', 'toml')
>>> tuple( readers.items( ) ) == tuple( zip( readers.keys( ), readers.values( ) ) )
True

Unions

The union operator (|) combines entries from two dictionaries or a dictionary and a mapping, creating a new dictionary. The operation maintains the accretive contract by preventing duplicate keys:

>>> formats = Dictionary( csv = csv_reader, json = json_reader )
>>> more_formats = Dictionary( yaml = yaml_reader, toml = toml_reader )
>>> all_formats = formats | more_formats
>>> all_formats
accretive.dictionaries.Dictionary( {'csv': <function csv_reader at 0x...>, 'json': <function json_reader at 0x...>, 'yaml': <function yaml_reader at 0x...>, 'toml': <function toml_reader at 0x...>} )

When operands have overlapping keys, an error is raised:

>>> conflicting = Dictionary( json = yaml_reader )
>>> formats | conflicting
Traceback (most recent call last):
...
accretive.exceptions.EntryImmutabilityError: Cannot alter or remove existing entry for 'json'.

Intersections

The intersection operator (&) can be used in two ways:

1. With another mapping to keep entries with matching key-value pairs:

>>> d1 = Dictionary( a = 1, b = 2, c = 3 )
>>> d2 = Dictionary( a = 1, b = 3, d = 4 )  # Note: b has different value
>>> d1 & d2  # Only entries that match exactly
accretive.dictionaries.Dictionary( {'a': 1} )

2. With a set or keys view to filter entries by keys:

>>> allowed = { 'a', 'b' }
>>> d3 = d1 & allowed  # Keep only entries with allowed keys
>>> 'c' in d3
False

Producer Dictionary

Producer dictionaries have an interface nearly equivalent to collections.defaultdict. The first argument to the initializer for a producer dictionary must be a callable which can be invoked with no arguments. This callable is used to create entries that are absent at lookup time. Any additional arguments beyond the first one are treated the same as for the simple dictionary. Most of their behaviors are the same as for the simple dictionary, except as noted below.

>>> from accretive import ProducerDictionary

Initialization

A common use case is to automatically initialize a mutable data structure, such as a list, and add elements or entries to it by merely referencing its corresponding key… without checking whether the entry exists or creating the entry first.

>>> watch_lists = ProducerDictionary( list )
>>> watch_lists
accretive.dictionaries.ProducerDictionary( <class 'list'>, {} )

Production of Absent Entries

>>> watch_lists[ 'FBI: Most Wanted' ]
[]
>>> watch_lists
accretive.dictionaries.ProducerDictionary( <class 'list'>, {'FBI: Most Wanted': []} )
>>> watch_lists[ 'Santa Claus: Naughty' ].append( 'Calvin' )
>>> watch_lists
accretive.dictionaries.ProducerDictionary( <class 'list'>, {'FBI: Most Wanted': [], 'Santa Claus: Naughty': ['Calvin']} )

Updates

>>> watch_lists.update( { 'US Commerce: Do Not Call': [ 'me' ] }, Tasks = set( ) )
accretive.dictionaries.ProducerDictionary( <class 'list'>, {'FBI: Most Wanted': [], 'Santa Claus: Naughty': ['Calvin'], 'US Commerce: Do Not Call': ['me'], 'Tasks': set()} )

Access of Absent Entries

The get method behaves the same as it does on the simple dictionary. I.e., it does not implcitly create new entries in a producer dictionary. This is the same behavior as collections.defaultdict.

>>> watch_lists.get( 'TSA: No Fly' )
>>> watch_lists.get( 'TSA: No Fly', 'Richard Reid' )
'Richard Reid'
>>> watch_lists
accretive.dictionaries.ProducerDictionary( <class 'list'>, {'FBI: Most Wanted': [], 'Santa Claus: Naughty': ['Calvin'], 'US Commerce: Do Not Call': ['me'], 'Tasks': set()} )

Copies

The copy method creates a new producer dictionary, which is initialized with the same producer and data as the dictionary on which the method is invoked.

>>> ddct1 = ProducerDictionary( lambda: 42, { 'foo': 1, 'bar': 2 }, orb = True )
>>> ddct1
accretive.dictionaries.ProducerDictionary( <function <lambda> at 0x...>, {'foo': 1, 'bar': 2, 'orb': True} )
>>> ddct2 = ddct1.copy( )
>>> ddct2
accretive.dictionaries.ProducerDictionary( <function <lambda> at 0x...>, {'foo': 1, 'bar': 2, 'orb': True} )

Comparison

Equality comparisons may be made against any registered subclass of collections.abc.Mapping. Note that the producer is excluded from the equality comparison; only data is compared; this is the same behavior as collections.defaultdict.

>>> ddct2 == { 'foo': 1, 'bar': 2, 'orb': True }
True

Validator Dictionary

Validator dictionaries ensure that all entries satisfy specified criteria. The first argument to the initializer must be a callable which accepts a key and value and returns a boolean indicating whether the entry is valid. Any additional arguments are treated the same as for the simple dictionary.

>>> from accretive import ValidatorDictionary

Let us illustrate this with a dictionary that only accepts integer values.

>>> numbers = ValidatorDictionary( lambda k, v: isinstance( v, int ) )
>>> numbers[ 'answer' ] = 42
>>> numbers[ 'pi' ] = 3
>>> numbers
accretive.dictionaries.ValidatorDictionary( <function <lambda> at 0x...>, {'answer': 42, 'pi': 3} )

Invalid entries are rejected.

>>> numbers[ 'e' ] = 2.718
Traceback (most recent call last):
...
accretive.exceptions.EntryValidityError: Cannot add invalid entry with key, 'e', and value, 2.718, to dictionary.

This includes attempts to add invalid entries via update.

>>> numbers.update( phi = 1.618 )
Traceback (most recent call last):
...
accretive.exceptions.EntryValidityError: Cannot add invalid entry with key, 'phi', and value, 1.618, to dictionary.

Producer-Validator Dictionary

Producer-validator dictionaries combine the behaviors of producer and validator dictionaries. The first argument must be a producer callable, and the second must be a validator callable. Any additional arguments are treated the same as for the simple dictionary.

>>> from accretive import ProducerValidatorDictionary

A common use case is to automatically initialize data structures of a specific type while ensuring that only those types can be stored.

>>> registries = ProducerValidatorDictionary(
...     list,
...     lambda k, v: isinstance( v, list )
... )
>>> registries
accretive.dictionaries.ProducerValidatorDictionary( <class 'list'>, <function <lambda> at 0x...>, {} )

The producer must create values that satisfy the validator.

>>> handlers = registries[ 'handlers' ]  # Produces new list
>>> handlers.append( 'default_handler' )
>>> registries[ 'plugins' ] = [ ]  # Valid manual assignment
>>> registries
accretive.dictionaries.ProducerValidatorDictionary( <class 'list'>, <function <lambda> at 0x...>, {'handlers': ['default_handler'], 'plugins': []} )

Invalid entries are rejected, whether assigned directly or via update.

>>> registries[ 'modules' ] = { }  # Not a list
Traceback (most recent call last):
...
accretive.exceptions.EntryValidityError: Cannot add invalid entry with key, 'modules', and value, {}, to dictionary.
>>> registries.update( callbacks = set( ) )  # Not a list
Traceback (most recent call last):
...
accretive.exceptions.EntryValidityError: Cannot add invalid entry with key, 'callbacks', and value, set(), to dictionary.

If the producer returns an invalid value, the entry is rejected.

>>> bad_registries = ProducerValidatorDictionary(
...     dict,  # Produces dictionaries
...     lambda k, v: isinstance( v, list )  # Requires lists
... )
>>> bad_registries[ 'anything' ]  # Production fails validation
Traceback (most recent call last):
...
accretive.exceptions.EntryValidityError: Cannot add invalid entry with key, 'anything', and value, {}, to dictionary.